The present invention relates to a hybrid scan type implantation apparatus in which an ion beam is electrically scanned and a target is mechanically scanned in the direction substantially perpendicular to the scanning direction of the ion beam.
FIG. 1 shows such a type of ion implantation apparatus as a prior art.
In this ion implantation apparatus, a spot shaped ion beam 2 is generated from an ion source after mass analysis, acceleration and so forth are conducted, if necessary. The ion beam 2 is electrostatically scanned in an X direction (for example, a horizontal direction) by a pair of scanning electrodes 4 to which scanning voltages (triangle wave voltages at around 1000 Hz, for example) are supplied from a scanning power supply 6 so that the ion beam 2 is plainly spread in the X direction.
On the other hand, a target 8 (a wafer, for example) is held by a holder 10 in the radiation area of the ion beam 2. The target 8 and the holder 10 are mechanically scanned by a drive unit 12 in a Y direction (for example, in the vertical direction) perpendicular to the X direction. This mechanical scanning operation is associated with the scanning operation of the ion beam 2 so as to equally implant ions into the entire surface of the target 8.
More specifically, at one end section of the scanning area in the X direction of the ion beam 2, a beam current measurement device (for example, a Faraday cup) 14 for measuring the beam current of the ion beam 2 is provided. In this prior art, the beam current I measured by the beam current measurement device is converted into a pulse signal by a converter 16 and the resultant pulse signal is output to a control unit 18. The control unit 18 computes the mechanical scanning speed of the target 8 according to the measured data and controls a drive unit 12 so that it drives the target at the computed speed. More specifically, the drive unit 12 is controlled so that it vertically drives the target 8 at a speed proportional to the beam current I.
In the drive unit 12, for example, a mechanism comprising a linear drive motor, or a combination of a rotation motor and a ball screw can be used.
In the above apparatus, the beam current I measured by the beam current measurement device 14 is as shown in FIG. 2. The points a and b in FIG. 2 correspond to the points a and b in FIG. 1, respectively.
Thus, the control unit 18 must carry out a process of detecting the amount of presently received beam current I, computing the speed of the target 8 necessary for the next reciprocative scanning operation, and outputting a drive signal DS to the drive unit 12 in a time period T1 from the measurement of the previous beam current I to the start of the next reciprocative scanning operation.
However, in the conventional ion implantation apparatus, the time period necessary for one reciprocative scanning operation is approximately 1 msec. The time period T1 is as small as several hundred micro sec. Thus, the control unit 18 should have the performance for executing the computation process described above in such a short time period. In addition, the time period necessary for the computation process varies depending on the amount of the beam current I (for example, as the beam current I increases, the number of process steps in the control unit 18 increases).
If the computation process time period in the control unit 18 exceeds the time period T1, the control operation of the scanning speed of the target 8 would be disordered and thereby the ion implantation operation would be incorrectly executed.
Thus, the control unit 18 must have such a performance that a time period necessary for the longest process is less than T1. However, the control unit 18 having such a high process speed would be very expensive or very difficult to manufacture.